In modern digital transmission systems, the data to be transmitted are combined in groups comprising a plurality of bits. A group comprising a plurality of bits, for example 3 bits in the mobile radio standard EDGE, is called a complex symbol. The complex symbol is supplied to a pulse shaping filter which produces a modulation signal in the form of a complex envelope therefrom. The modulation signal can be represented using Cartesian coordinates in the formm(k)=I(k)+jQ(k),where the component I(k) is called the real component and Q(k) is called the quadrature component of a baseband signal.
To produce the complex envelope, a modulator is often used, as shown schematically in prior art FIG. 6. In this case, the two components I(k) and Q(k) of the signal m are in the form of digital signals and are supplied to the inputs 2a and 2b. Digital/analog converters 3a convert the two digital components I(k) and Q(k) into analog signals I′(k) and Q′(k) and supply them via low-pass filters 3b for rejecting harmonic components to an “I/Q modulator”. Using two step-up frequency converters 3c, the analog components I′(k) and Q′(k) are converted to an output signal at the frequency RF and are output at the output 2c. 
Advances in circuitry now make it possible to provide frequency and phase modulators which can be directly digitally actuated by suitable phase locked loops. An example of such a phase locked loop with direct digital actuation for phase and frequency modulation is described in German patent application DE 10255863.9.
The direct actuation of a phase locked loop for phase and frequency modulation means that it is now possible to use polar modulators for producing radio-frequency modulated output signals instead of the I/Q modulators used to date. A symbol can be represented as a complex modulation signal using polar coordinates in the formm(k)=A(k)exp(jφ(k)).
From the form of the complex baseband signal using polar coordinates which is shown above, it can be seen that a polar modulator modulates both the amplitude A(k) and the phase φ(k).
Phase modulation using the phase locked loop specified in DE 10255863.9 is possible without any great complexity.
In addition, however, amplitude modulation is also necessary. In this case, it should be remembered that the previously used polar modulators are particularly sensitive to errors in the amplitude path of the polar modulator, these being called offsets. The offsets produce additional signal components in the output signal which can result in an increased error rate during data transmission. Their cause is frequently unwanted DC components, for example as a result of leakage currents, which add a usually constant signal to the amplitude modulation signal.